1. Field of the Invention
The present invention relates to a structure of a solar cell well suited for a power source of electric instrument with low power consumption, such as a calculator and a watch, and also relates to a method of fabricating the same.
2. Description of the Related Art
Solar cells are becoming popular not only for a sunlight electrical generating system, which is installed outdoors, but also for a power source of electric instrument with low power consumption, such as a calculator, a radio, and a watch. In such a consumer use, for example, like a wrist watch, in the case where importance is attached to not only the function but also to its external design, a mounting method of the solar cell is also devised. The solar cell is directly used as a face of the watch, or is installed under a semitranslucent face of watch to make it unnoticeable.
In most of solar cells in the consumer use, glass, stainless, organic resin material or the like is used for a substrate, and a photoelectric conversion layer is formed thereon with a thin film of amorphous semiconductor, microcrystalline semiconductor, or chalcopalide-based (or II-VI group) compound semiconductor. Especially, the solar cell using an organic resin material for the substrate is thin and lightweight, and has an excellent shock resistance so that it is not cracked even if it is dropped. Accordingly, it is suitable for the solar cell to be mounted to a portable product, such as a card type calculator, a wrist watch, and a remote-control device of an indoor electric instrument such as a television.
In the solar cell fabricated by using the organic resin material for the substrate, there is a well known technique for forming a photoelectric conversion layer with a non-monocrystalline semiconductor material, such as amorphous silicon or microcrystalline silicon, fabricated by a plasma CVD method. As means for increasing the productivity of such a solar cell and decreasing the manufacturing costs, there is known a roll-to-roll method in which a long organic resin film substrate wound into a roll shape or a metal film substrate of a stainless alloy etc., is fed out from one side of substrate holding means provided in a plasma CVD apparatus, a sputtering apparatus, or another manufacturing apparatus, the substrate is made to continuously (or stepwise) move in a treatment space of coating formation or the like, and it is rewound by substrate holding means provided at the other side.
In addition, there is known a method in which a rear electrode opposite to a transparent electrode provided at an incident side of light is formed by printing a paste containing a powder of conductive material with a binder of organic resin material, instead of a metal film by a sputtering method, a vacuum evaporation method, etc. Japanese Patent No. 2698401 discloses a technique in which a rear electrode of a solar cell is formed by a printing method using a conductive paste containing a conductive material of molybdenum powder with a binder of phenolic resin.
As a method of connecting a circuit substrate of a calculator, a watch, a remote-control device of an indoor electric instrument, or the like to an output terminal of a solar cell, in addition to soldering or a method of thermocompression bonding of a flexible printed substrate by an anisotropic conductive adhesive, a pressure contact system using a spring terminal is adopted. Although this method can prevent damage by heat from being applied to the solar cell, if the output terminal is formed of a metal material, similarly to the rear electrode, there has been a problem of aging of contact resistance due to surface oxidation or the like. Thus, contrivance has been made such that a carbon electrode is intentionally provided at this portion.
Since a conductive carbon electrode film obtained by applying a carbon paste by a printing method, drying and hardening, is not oxidized, the aging of the contact resistance to a spring terminal is small. Therefore, it is regarded as a suitable material. However, there has been a problem in that the contact resistance to a semiconductor layer is high, and peeling and warp of a substrate, etc. would occur.
On the other hand, as another problem, like a consumer use electric instrument, in the case where importance is mainly attached to photoelectric conversion characteristics under low illumination in an indoor environment or the like, a minute short circuit region is formed between a transparent electrode and a rear electrode by electro-static damage, and output voltage is lowered. As a result, the reliability of a product has been remarkably deteriorated.
The present invention is a technique for solving the foregoing problems, and an object thereof is to provide a solar cell in which stability at a connection portion between a circuit substrate of an electric instrument and a rear electrode, and reliability against electro-static damage are improved, and also to provide a method of fabricating the same.
In order to achieve the object, according to the present invention, in a solar cell using an organic resin material for a substrate, a rear electrode is formed of a material containing carbon as a main ingredient. In formation of the rear electrode, a thermosetting conductive carbon paste is used and the formation is made by a printing method.
A conventional conductive film formed by mixing and dispersing a powder of conductive material on a flexible substrate using a binder of organic thermoplastic resin is weak to an organic solvent, and at the time of formation of an insulating sealing resin layer formed thereon, expansion and dissolving of the resin component by the solvent occurs. Therefore, it has not been capable of being used. Besides, in an environment resistance test of the conductive film itself, a matrix component of the resin easily receives the influence of the change of temperature and humidity, and softened resin is apt to intervene between contacted conductive fine powders, and as a result, series resistance is increased. Consequently, it has not been capable of withstanding the environment resistance test.
Therefore, a resin binder in view of the following points is used for the conductive film of the thermosetting conductive carbon paste of the present invention. First, conductive fine particles are highly filled to raise conductivity of the film, and at the same time, the molecular structure of a matrix resin component of the film and a crosselinking agent are optimized to obtain the conductive electrode film having high heat resistance and humidity resistance and the matrix resin of sufficiently high crosslinking strength. Even when the upper portion of this electrode film is covered with, for example, an insulating ink film, it has such solvent resistance that it sufficiently withstands the contained solvent. Besides, with respect to the conductive contact at the interface between the conductive fine particle and a transparent thin film conductive film as well, the resin matrix is not swelled and dissolved, and can be firmly fixed. Further, against the increase of temperature and humidity as well, since the heat resistance and humidity resistance of the resin matrix is improved, the change of mechanical properties is small. As an example of a method of obtaining a conductive coating film including such resin matrix, a saturated polyester resin having the highest possible residual hydroxyl group content is used, and in order to keep the pot life of a curing agent having high reactivity with this functional group, that is, a conductive ink more stably in a period including the time of screen printing, such resin blending composition is designed that glass transition temperature (Tg) becomes at least 70xc2x0 C. or more by high density thermal crosslinking using the curing agent, such as multi-functional block isocyanate compound which is made inactive under room temperature by a block agent and easily dissociates isocyanate especially at low temperature, or melamine resin. More specifically, the thermosetting conductive paste is formed such that its main component is a conductive coating matrix resin typified by saturated polyester resin in which a dispersion property enabling conductive carbon fine particles to be highly filled, a printing property, and conductive coating formation performance thereafter are excellent, consideration is sufficiently paid to monomer blending at the time of condensation polymerization of dicarboxylic acid and diol, an OH value content is high, and Tg is higher than that of the resin, and a resin binder is a combination with block isocyanate which easily dissociates highly reactive isocyanate groups by low temperature heating.
When the fine granular conductive carbon particles are obtained by mixing artificial graphite of an average particle diameter of 0.1 to several tens of xcexcm into the carbon particles at a mixing ratio of 1/3 to 3/1 in weight ratio and by further making the particles fine by a ball mill or the like, it is effective in decrease of the electric resistance of a carbon conductive coating film.
The organic resin material is generally apt to be charged, and the electrostatic withstand voltage of a solar cell formed thereon becomes deteriorated. In order to improve the electrostatic withstand voltage between terminals, the resistance of the transparent electrode and the rear electrode is rather made high so that the electric field is not concentrated on the element portion. The sheet resistance of the conductive film using the thermosetting conductive carbon paste and formed by the printing method is 30 to 80 xcexa9/xe2x96xa1, and it is possible to make the resistance just coincident with the value of the transparent electrode formed of ITO.